BACKGROUND Patient beliefs in the effectiveness of treatment may have an important influence on treatment outcome. METHODS Associations between patient beliefs in the credibility of treatment and outcome were explored in a randomised controlled trial of major depression in primary care (n=155). The four treatments were antidepressant medication given by research general practitioner, problem solving treatment given by research general practitioner or research practice nurse over 12 weeks or a combination of problem solving treatment and antidepressant medication. Patients' belief in the credibility of treatment was assessed using a brief Credibility Scale, that was completed following randomisation and after treatment. Depression outcome was measured at 6, 12 and 52 weeks using the Hamilton Rating Scale for depression, and the Beck depression inventory. RESULTS Pre-treatment, medication treatment was associated with a higher certainty of recovery than was problem-solving treatment from the nurse (p=0.018). Post-treatment, medication and combination treatment were seen as more logical than problem-solving treatment from the nurse (p<0.03). Post-treatment medication had higher certainty of recovery and was more highly recommended to a friend. Linear regression demonstrated that the depression outcome measures were not associated with either pre- or post-treatment credibility. CONCLUSIONS Patients found all four treatments highly credible following their initial explanation. There was a significant difference both pre- and post-treatment in favour of patients finding treatment involving medication more credible than problem-solving from a nurse. Pre- and post-treatment scores of credibility were not associated with outcome.